As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which exhibit high energy density and operating potential, and have long cycle lifespan and a low self-discharge rate, are commercially available and widely used.
In addition, as interest in environmental problems is increasing, research into electric vehicles (EVs), hybrid electric vehicles (HEVs), and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is actively underway. As a power source of EVs, HEVs, and the like, a nickel-metal hydride (Ni-MH) secondary battery is mainly used. However, research into lithium secondary batteries having high energy density, high discharge voltage, and high output stability is actively carried out and some of the lithium secondary batteries are commercially available.
A lithium secondary battery has a structure in which an electrode assembly, which includes a positive electrode prepared by coating a positive electrode active material on a positive electrode current collector, an negative electrode prepared by coating an negative electrode active material on an negative electrode current collector, and a porous separator disposed between the positive electrode and the negative electrode, is impregnated with a lithium salt-containing non-aqueous electrolyte.
As positive electrode active materials for such lithium secondary batteries, lithium-containing cobalt oxides such as LiCoO2 are mainly used. In addition thereto, use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure and the like, and lithium-containing nickel oxides such as LiNiO2 is also under consideration.
LiCoO2 is widely used due to excellent overall physical properties such as excellent cycle properties, and the like, but is low in safety. In addition, due to resource limitations of cobalt as a raw material, LiCoO2 is expensive and mass use thereof as power sources in fields such as electric vehicles and the like is thus limited. Due to characteristics of preparation methods of LiNiO2, it is difficult to mass-produce LiNiO2 at reasonable expense.
On the other hand, lithium manganese oxides, such as LiMnO2, LiMn2O4, and the like, are advantageous in that they contain Mn, which is an abundant and environmentally friendly raw material, and thus are drawing much attention as a positive electrode active material that can replace LiCoO2. However, such lithium manganese oxides also have disadvantages such as poor cycle characteristics and the like.
First, LiMnO2 has disadvantages such as a low initial capacity and the like. In particular, LiMnO2 requires dozens of charge and discharge cycles until a constant capacity is reached. In addition, capacity reduction of LiMn2O4 becomes serious with increasing number of cycles, and, at particularly high temperature of 50° C. or more, cycle characteristics are rapidly deteriorated due to decomposition of an electrolyte solution, elution of manganese and the like.
Meanwhile, as lithium-containing manganese oxides, there is Li2MnO3 in addition to LiMnO2 and LiMn2O4. Since structural stability of Li2MnO3 is excellent but it is electrochemically inactive, Li2MnO3 itself cannot be used as a positive electrode active material of secondary batteries. Therefore, some prior technologies suggest technology of using a solid solution of Li2MnO3 and LiMO2 (M=Co, Ni, Ni0.5Mn0.5, Mn) as a positive electrode active material. In such a positive electrode active material solid solution, Li and O are separated from a crystal structure at a high voltage of 4.4 V and, thus, electrochemical activity is exhibited. However, there are problems such as high possibility of electrolyte solution decomposition and gas generation at high voltage, and mass use of relatively expensive materials such as LiMO2 (M=Co, Ni, Ni0.5Mn0.5, Mn) and the like.
In addition, lithium-containing manganese oxides have low conductivity, and thus, have large difference in resistance, depending upon particle sizes. In addition, when activation thereof is performed at a high voltage of 4.4 V or more in order to exhibit high capacity, Li2O is separated from Li2MnO3, and thus, structural change may occur. Furthermore, due to structural characteristics of lithium-containing manganese oxide crystals, it is difficult to guarantee desired stability and it is limited to anticipate improved energy density.
In addition, carbon based materials are mainly used as negative electrodes for lithium secondary batteries. However, the carbon based materials have a low electric potential of 0 V with respect to lithium, and thus, cause reduction of an electrolyte, thereby generating gas. To address this problem, metal oxides having a relatively high electric potential are used as a negative electrode active material.
However, in this case, a large amount of gas is generated during activation and charge-discharge processes, and thus, side reaction of an electrolyte is facilitated, thereby decreasing secondary battery safety.
Therefore, there is a need to develop a technology that can resolve the above-described problems.